This invention relates to mode control in solid state lasers and, more particularly, to such lasers which favor operation in either the transverse electric (TE) mode or the transverse magnetic (TM) mode and discriminate against the other.
On form of solid state laser is the buried heterostructure (BH) semiconductor laser which is the optical source of choice in most lightwave communications systems, especially long haul transmission systems operating at high bit rates (e.g., above 1 Gb/s). Typically BH lasers operate in the fundamental TE mode because the waveguide characteristics of the active layer (asymmetric cross-section and refractive index relative to that of the cladding layers) tend to support operation in a TE mode and suppress operation in a TM mode.
The suppression of the TM modes, however, is incomplete; during operation of the laser in a TE mode, occasionally the laser will switch to operation in a TM mode or in both a TM and a TE mode. The random occurrence of the TM mode is a statistical process and represents a form of noise; each occurrence of a TM mode may constitute an error in systems incorporating the laser. The number of such errors is measured, in digital systems, by the number of pulses which are incorrectly detected as logical "1" when in fact a logical "0" was transmitted, or conversely. The number of such erroneously detected pulses is known as the bit error rate (BER).
Generally, the higher the system bit rate, the lower the BER which may be required. Thus, for example, at a system bit rate of 1.7 Gb/s the required BER may be as low as 10.sup.-14 ; i.e., only one error in 10.sup.14 pulses occurs.
Stringent BER criteria have an adverse effect on the ability to manufacture semiconductor lasers with acceptable yields; the lower the BER, the lower the yield, in general.
Thus, there is a need for a semiconductor laser design in which TM modes are sufficiently suppressed to enable lasers with low BERs to be manufactured with acceptable yields.